The present invention relates to systems and methods for reducing speckle in a scanned image. More specifically, the present invention relates to the design and methods of operation of a visible light source and laser projection system to reduce the presence of speckle that may be visible in a laser projection image. Speckle that is present in the image may seriously degrade the image quality. Speckle may result whenever a coherent light source is used to illuminate a rough surface, for example, a screen, or any other object that produces a diffused reflection or transmission.
Particularly, a multitude of small areas of the screen or other reflecting object scatter light into a multitude of reflected beams with different points of origination and different propagation directions. At an observation point, for example in the observer's eye or at the sensor of a camera, these beams may interfere constructively to form a bright spot, or destructively to form a dark spot, producing a random granular intensity pattern known as speckle. Speckle may be characterized by the grain size and contrast, usually defined as a ratio of standard deviation to mean light intensity in the observation plane. For a large enough illuminated area and a small enough individual scattering point size, the speckle will be “fully developed,” with a brightness standard deviation of 100%. For example, if an image is formed on the screen using laser beams, such granular structure will represent noise, or a serious degradation of the image quality.
The general concept of using diffusers to minimize speckle consists of projecting an intermediate image over a small sized diffusing surface, and using projection optics to project that intermediate image over the final screen. By moving the small size diffuser, the phase of the electric field is scrambled over time, which results in changing the perceived speckle pattern. If the diffuser is moving fast enough, the perceived speckle pattern changes at high frequencies and are averaged in time by the eye. To work efficiently, multiple speckle frames need to be created over the integration time of the eye, which is typically in the order of 50 Hz. This concept can easily be applied to projection systems that display images on a frame-per-frame basis.
However, it is much more difficult to incorporate a diffuser to reduce the perceived speckle in a laser scanning projector system because the scanned spot is moving extremely fast and stays at approximately the same location over durations in the order of 40 nS. It is very difficult to display multiple speckle patterns over such a small duration because the maximum speed at which different speckle patterns can be displayed is the frequency at which full frames are being displayed.